The veteran tug Liz Vinik, 64 years old and 3,000 HP, headed up the North River Thursday morning with A&S Transportation sludge barge Maria. The barge would be loading treated residual solid waste at the North River sewage plant and ferrying it to the Passaic Valley plant in Newark for dewatering there. The city normally employs its own fleet of small tankers for this work but for the past six months or so we have seen privately operated barges occasionally servicing the plant, which might perhaps be a result of two of the city’s five tankers being out of service.